In order to measure and characterize engine exhaust particulate matter (PM) emissions, it is required that the engine exhaust be diluted. More specifically, because the exhaust contains a high concentration of particles, the measurement sample must be diluted (mixed with dilution air) with a high dilution ratio (typically greater than 100:1) to reach the measurable range of particle number instruments. Once the measurement is made from the mixture, the dilution ratio can be used to determine the actual particle concentration in the sample.
In addition to providing high dilution ratios, an emissions measurement system should provide a wide range of ratios. Due to the variety of engine technologies and operating conditions, the concentration of engine exhaust particles may vary widely. To obtain accurate measurement results, wide range dilution capability is required.
One approach to particle measurement is to use a partial flow diluter that controls dilution air flow and total mixture flow with mass flow controllers. The sample flow is calculated by subtracting the dilution air flow from the total mixture flow. Then, the dilution ratio is calculated by dividing the total mixture flow by the calculated sample flow. At low dilution conditions, this approach provides an accurate dilution ratio calculation. However, as the dilution ratio increases, the accuracy of the calculated dilution ratio suffers due to uncertainties of the total flow and dilution air flow measurements. This results in inaccurate characterizations of exhaust emissions.
For emissions measurements at higher dilution ratios, ejector diluters have been used. However, ejector diluters tend to have a narrow dilution ratio range, and have other shortcomings.
Rotating disk diluters permit a wide range of dilution ratios, but tend to exhibit dilution ratio drift and other shortcomings.
U.S. Pat. No. 7,201,071, to Wei, et al, describes a wide range continuous diluter. A flow meter in the sample line directly measures the sample flow without causing particle losses.